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ATOMIC STRUCTURE NOTES
Science Exams Sorted

2020
www
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co
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Let us start with that first question
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And secondly, what is an atom made of?
Atoms are made of three different types of
particles
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Protons
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Neutrons
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Electrons
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Electrons are located in the orbits also known as electron shells surrounding the nucleus
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Take the example
of helium below
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4

Mass number
Tells us the number of protons and neutrons in an
atom
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The number of neutrons can be found by subtracting the atomic number from the mass
number
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Ca

20
40

b
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O

Protons =

Protons =

Protons =

Electrons =

Electrons =

Electrons =

Neutrons =

Atomic Structure Notes

Neutrons =
2

8
16

Neutrons =

Science Exams Sorted

Atomic Structure Notes
Electronic configuration
As mentioned, electrons are arranged in orbits or shells around the nucleus of an atom
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They are as follows;

First shell

→

a maximum of 2 electrons
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Third shell

→

a maximum of 8 electrons
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Electronic Configuration
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Dot & Cross Diagrams
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For example;

N

Nitrogen
2+5=7
2 in 1st shell
5 in 2nd shell

7
14

configuration = 2, 5

With Dot and Cross diagrams electrons are represented by Dots or Crosses to show electrons,
and circles to show the shells
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So, an isotope can still be the same element even though the mass numbers are different
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Take the next figure as an example
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Having the same number
of protons and a different number of neutrons �
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“Two isotopes” are the most common style of question
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Take the following example
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g
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Calculate
the relative atomic mass of the sample of copper
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6

The existence of isotopes results in relative atomic masses of some elements not being whole
numbers

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